My Answer: Both ab exercises and cardio are very taxing for the ectomorph, and they are taxing for very different reasons. Ab exercises work your nervous system pretty hard, since there are a huge bundle of nerves in the abdominal area. So you're likely to overtrain if you include too much ab work. The ectomorph has to avoid overtraining, so that he doesn't compromise his growth. Besides, if you're an ectomorph, then you should have a thin waist anyway that's going to be worked indirectly by squats.
But if you must do an ab exercise, then do some hanging leg raises
As far as cardio, it will overtrain you metabolically. If you're an ectomorph and already skinny, then why waste your recuperative abilities doing cardio? Your goal isn't fat loss, it's muscle building. But if you have to do cardio, then do some intermittent form of cardio, like kickboxing or wrestling.
Be warned: you will not gain as much muscle as you'd like if you add ab work and cardio.